This invention relates broadly to methods and tools for measuring formation geomechanical parameters as a function of both depth and azimuth, whereby features of the formation are determined.
Acoustic measurements can be used to detect the presence of mechanically damaged rock in stressed rock around boreholes prior to the collapse of the borehole.
The art of sonic well logging for use in determining formation parameters is a well established art. Sonic well logs are typically derived from sonic tools suspended in a mud-filled borehole by a cable. The tools typically include a sonic source (transmitter) and a plurality of receivers which are spaced apart by several inches or feet. Typically, a sonic signal is transmitted from the transmitter at one longitudinal end of the tool and received by the receivers at the other, and measurements are made every few inches as the tool is drawn up the borehole. The sonic signal from the transmitter or source enters the formation adjacent the borehole, and the arrival times of the compressional (P-wave), shear (S-wave) and Stoneley (tube) waves are detected by the receivers. The receiver responses are typically processed in order to provide a time to depth conversion capability for seismic studies as well as for providing the determinations of formations parameters such as porosity. It has long been known that the drilling of a borehole into a formation disturbs the stress field that was present in the formation prior to the existence of the borehole. The drilling of the borehole results in circumferential and radial stress concentrations around the borehole, where the resulting stress field is strongly anisotropic at the borehole wall, but the effects of the borehole decrease rapidly with distance into the formation. It has also been established that acoustic velocities in rock are sensitive to applied stress, with both compressional and shear velocities increasing with hydrostatic stress. Uniaxial stress produces compressional and shear wave anisotropy and shear wave birefringence (velocity dependent on polarization). These results have been related by A. Nur, xe2x80x9cEffects of Stress on Velocity Anisotropy in Rocks with Cracksxe2x80x9d, Journal Geophysics. Res.; Vol. 76, 8, p. 2022(1971), and by D. L. Anderson et al., xe2x80x9cThe Effect of Oriented Cracks on Seismic Velocitiesxe2x80x9d, Journal Geophysics Res.; Vol. 82 p.5374 (1974), to stress-induced anisotropy of microcrack orientations. U.S. Pat. No. 5,544,127, issued Aug. 6, 1996, to Winkler, a co-inventor of the present invention, discloses the use of a sonic borehole tool to measure velocity around the borehole as a function of azimuth. In this patent Winkler teaches that formation properties can be determined from a knowledge of velocity as a function of azimuth, and that the azimuthal direction of minimum velocity around the borehole predicts the propagation direction of artifically induced hydrofractures. He further teaches that sonic velocity variation around the borehole at a particular depth of the borehole may be taken as an indication of susceptibility to failure, with higher velocity variations indicative of a more poorly consolidated formation or a formation with a large uniaxial stress. He further teaches that the curvature of the velocity versus stress curve in the formation is indicated by how poorly a sine wave fits to the velocity data. He further teaches that other parameters of the formation may be obtained by fitting a best fit curve to the azimuth versus velocity data, where adjustable parameters of the best fit curve constitute the formation parameters.
Acoustic measurements are better able to detect mechanical damage than are other logging techniques based on resistivity, density, radioactivity, or magnetic resonance because the acoustic waves stress and strain the solid granular structure of the rock.
The invention provides a method for locating and measuring mechanical damage in rock surrounding a borehole by using one or both of reductions in ultrasonic compressional wave velocity in the rock as a function of azimuth, and increase in focused acoustic energy in the rock associated with local increase in ultrasonic compressional wave amplitude resulting from velocity gradients. A first preferred embodiment uses a combination of azimuthal ultrasonic compressional wave velocity data and azimuthal ultrasonic compressional wave energy data. A second embodiment uses azimuthal ultrasonic compressional wave velocity data and omni-directional sonic velocity data, with a comparison test or a curve fitting test. A third embodiment uses azimuthal ultrasonic compressional wave energy data.